Ariana Grande has a larger-than-life voice…and an ego to match! While filming a TV special in mid-August, crew members say, the 21-year-old “Problem” singer was impossible to work with.

“She was a total nightmare,” explains a source on the set. “She demanded to be shot only on her left side and told the lighting crew that if they used red light, she would ‘kill herself.'”

Producers at the venue were so disgusted with the way she treated everyone, they almost pulled the plug! “They were saying that she is, hands down, the most difficult person they have ever had to work with,” the insider adds.

And don’t even think of comparing her to another famous diva. “Ariana was mad that Mariah [Carey] dissed her in an interview, and it makes her furious to even hear Mariah’s name,” the source says.

Evidently, the singer’s awful attitude wasn’t a one-time thing — she didn’t make any friends on the set of a recent photo shoot, either. “She arrived hours late, with no explanation, and refused to do anything that the photographer asked,” an exasperated insider dishes. “She was acting like a crazy person. One minutes he was all sweetness…then she’d storm out, only to come back two minutes later, all smiles. It was very bizarre.”

While many young It Girls are obsessed with their looks, Ariana takes her vanity to extreme levels. She’s apparently terrified to be seen without layers of makeup and her hair extensions: she even refused to do the popular ALS ice bucket challenge! “Celebrities all over the world are doing it for a good cause, but she doesn’t care,” the source adds. “Her skin is a big problem for her and she absolutely does not want to be seen barefaced.”